Generally, a dispenser cathode is classified into cavity reservoir type dispenser cathode, impregnated type dispenser cathode and sintered type dispenser cathode according to the structure and they have a common characteristics of a high density beam current and a longer lifetime. However, these dispenser cathodes are disadvantageous in that they have difficulty in applying to the electron tube such as cathode ray tube because they are operated at high temperature of 1100.degree. to 1200.degree. C.
That is to say, these dispenser cathodes should have a heater of a large calorific value because a large amount of thermal energy is needed in order to generate sufficient thermoelectron emission, and their parts should be made of materials accompanied by no thermal deformation due to the high temperature of heat.
Further, the neighboring parts of the cathode such as control grid and screen grid, and support means of the cathode should be made of heat-resistance material. Therefore, steady researches and developments have been made in order to solve the above problems.
For instance, Japanese patent laid open publication No. 86-13526A describes scandium impregnated type cathode with a low operating temperature of 800.degree. to 900.degree. C. As shown in FIG. 1, this impregnated type cathode is featured in that the thin film layer 1a containing W-Sc.sub.2 O.sub.3 is formed on the surface of the porous metal base body 2a impregnated with electron emissive material. But this impregnated type dispenser cathode has other problems such that it has unstable thermoelectron emission caused by a non-uniform distribution of Sc.sub.2 O.sub.3 and has adverse effect caused by the reaction of Ba oxide with Sc oxide. When Ba oxide is reacted with Sc oxide, Ba.sub.3 Sc.sub.4 O.sub.9 is produced as a by-product and coagulated locally on the surface of the porous metal body 2a, which then partly suppresses emission of the thermoelectrons and causes the emissive state of the thermoelectrons to be unstable.
Furthermore, the heat transfer is hindered because the thin film layer containing W-Sc.sub.2 O.sub.3 is formed on the surface of the porous metal base body impregnated with electron emissive material, and thus the production of scandium tungstate is delayed, and the aging time, i.e., the time required for forming monatomic layer containing Ba-Sc-O on the electron emissive surface becomes very longer, resulting in the decrease of the productivity. Meanwhile, the thin film layer containing W-Sc.sub.2 O.sub.3 is apt to damage by ion bombardment during operation of the cathode and thus the current density becomes suddenly decreased by loss of a monatomic layer, thereby shortening the lifetime greatly.